Conventionally, there are various protection relay devices for protecting a power system, and ground fault overvoltage relay devices are often adopted to detect a ground fault accident in a resistance grounding system (or an ungrounded system). A ground fault overvoltage relay device acquires respective phase voltages from a voltage transformer (VT) connected to the system, and detects an accident based on a zero-phase voltage derived from the respective phase voltages.
Generally, a fuse is connected to a secondary circuit of the voltage transformer, to protect the voltage transformer from an overcurrent caused by a short circuit failure in the secondary circuit or the like. When the fuse blows and the secondary circuit is disconnected, the ground fault overvoltage relay device cannot receive respective correct phase voltages. Accordingly, the ground fault overvoltage relay device may incorrectly determine that a ground fault accident has been detected, and thereby incorrectly output a break command to a breaker or the like. Thus, techniques for preventing incorrect determination due to a disconnection in the secondary circuit of the voltage transformer have been disclosed.
Japanese Patent Laying-Open No. H02-46128 (PTD 1) discloses a ground fault overvoltage relay configured to acquire phase voltages from a secondary circuit of a voltage transformer connected to a power system, and detect a ground fault based on the magnitude of a zero-phase voltage obtained by synthesizing the respective phase voltages. The ground fault overvoltage relay includes a first determination unit configured to detect that at least two phases of line voltages obtained from the respective phase voltages are less than or equal to a predetermined value, a second determination unit configured to detect that a ratio between a minimum value of the line voltages and a minimum value of the phase voltages is less than or equal to a predetermined value, and lock means configured to lock a ground fault detection output when the first or second determination unit has a detection output.